1. Field of the Invention
The present invention relates to a preform production apparatus, for producing preforms from molten glass in a step of producing an optical element, for example, and to a method of producing the same.
2. Related Art
In recent years, reductions in size and weight has progressed in the field of optical apparatuses, and aspherical lenses made of glass have often been used for the purpose of decreasing the number of lenses constructing optical systems of optical apparatuses. Aspherical lenses made of glass are produced mainly by precision press molding, i.e. press-molding a heated and softened glass preform (hereinafter referred to as “preform”) by use of a mold with a highly precise molding face, thereby transferring the shape of the molding surface of the mold to the preform.
This method provides advantages such as optical elements being able to be mass-produced in a short period of time while maintaining their highly precise dimensions, since optical lenses are shaped through a preform from molten glass, compared to conventional methods of producing optical lenses from plate glass through multiple steps such as cutting, processing, press, grinding and polishing.
There are a variety of methods to produce preforms for precision press molding; in recent years, float-shaping methods are frequently employed in which molten glass flowing is discharged from a nozzle, the molten glass flowing is received by a mold, and then float-shaped over a porous mold (see Patent Document 1). This method has been tried for various optical glasses since this method can produce preforms having a smooth surface in a short time and a high yield without cold-working such as cutting and polishing.
However, in recent years, the float-shaping has tended to be difficult to apply to high refractive index glasses, which are required as a material of aspherical lenses, etc. That is, optical glass having a special optical constant often exhibits an excessively low viscosity at the temperature where the glass can flow out without generating devitrification (so called, low-viscosity glass material), which makes it difficult to adjust shape using float-shaping with gas.
In order to solve the problems, a publicly known method holds molten glass flowing once on a support member, before transferring to a mold, to cool until a viscosity suitable for shaping is achieved, and then transfers thereof to a mold (see Patent Document 2).
However, in recent years, the viscosity of molten glass has been further decreasing and discharge velocity from nozzles has been of increasing significance, which requires very complicated motions, out of the assumption in conventional apparatuses, such as a plurality of support members preliminarily stands by per discharge nozzle, receives glass flow in series, and immediately moves at a high velocity or with changing the velocity to make the receiving site available to another support member. In a trend to require these complicated motions, a system has been demanded in which a degree of freedom in motion of support members considerably increases, the occupying area of support members decreases, and a number of support members can coexist in an apparatus.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-97023
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2006-265085